This invention relates generally to bandgap reference circuits, and more particularly, to switch capacitor bandgap reference circuits. A stable reference voltage is a requirement in almost all integrated circuits (IC). The typical requirement is that the reference voltage be stable as a function of temperature. This requires that the reference voltage circuit have a low temperature coefficient. A typical application for a low temperature coefficient reference voltage is as a reference voltage for a voltage regulator.
The most common reference voltage is the so-called "bandgap" reference voltage. One popular embodiment of a bandgap reference circuit is shown in FIG. 1. The circuit shown in FIG. 1 is known as the Brokaw bandgap cell, named after the inventor. The bandgap circuit of FIG. 1 includes two transistors Q1 and Q2 whose sizes and/or bias currents are properly ratioed so as to produce a corresponding base to emitter junction voltage. The circuit produces a voltage across resistor R1 equal to the difference between the base to emitter voltages of the transistors Q1 and Q2, i.e., V.sub.BE2 -V.sub.BE1. It can be shown that this voltage is proportional to absolute temperature (PTAT). If the circuit resistors have very low temperature coefficients, the currents flowing through the resistors R1 and R2 are also PTAT. The current through resistors R1 and R2 produce a voltage V.sub.R that is also PTAT. The voltage V.sub.BE1 across the base to emitter junction of transistor Q1 can be shown to be complementary to the absolute temperature (CTAT). By properly choosing the device sizes, the bias currents, and the resistor values, the reference voltage V.sub.o can be made approximately stable with temperature due to the two countervailing voltages. A more detailed discussion of this circuit can be found in A. Paul Brokaw, "A Simple Three Terminal Bandgap Reference," IEEE J. Solid-State Circuits, Vol. SC9, pp. 288-393, Dec. 1974.
The Brokaw bandgap reference circuit uses two transistors to generate a voltage that is proportional to absolute temperature. The use of two transistors, however, introduces a major source of error in the accuracy of the reference voltage. This error is due to the mismatch between the two transistors. To compensate for this mismatch it is often necessary to modify the resistive elements of the bandgap reference circuit by "trimming" the resistors to produce the desired reference voltage. Although trimming can be successfully performed, it increases the cost of manufacturing the IC.
A single transistor bandgap reference that does not suffer from the transistor mismatch problem is shown in FIG. 2. The single transistor bandgap reference circuit of FIG. 2 is described in U.S. Pat. No. 5,059,820 issued to Alan L. Westwick. The bandgap reference of FIG. 2 uses two switches to time division multiplex two current sources (I1 and I2) to a single bipolar transistor to achieve an output voltage reference that is, to a first order, independent of temperature. The circuit operates in one of two repeating modes, a "precharge" mode and "valid output reference" mode. During the precharge mode, clocks 1 (.PHI.1) and 3 (.PHI.3) are at a logic high and clock 2 (.PHI.2) is a logic low. Thus, during the precharge mode, switches S2, S3, and S5 are closed and switches S1 and S4 are open. In contrast, during the valid output reference mode, clock 2 (.PHI.2) is at a logic high and clocks 1 (.PHI.1) and 3 (.PHI.3) are at a logic low. Accordingly, during the output reference mode switches S1 and S4 are closed and the others are open.
During the precharge mode, the current produced by current source I1 is coupled to the transistor Q3 which develops a voltage V.sub.BE1 across the base emitter voltage. Capacitors C1 and C2 precharged during this time and the output voltage VOUT goes to zero. During the valid output reference mode the current produced by current source I2 is supplied to the transistor Q3 and a base-to-emitter voltage V.sub.BE2 is produced. A PTAT voltage .DELTA.V.sub.BE is developed during the output reference mode and the output of the differential amplifier A1 assumes a value which is the sum of the scaled .DELTA.V.sub.BE and a scaled V.sub.BE1. The output reference voltage VOUT is therefore given by the equation: EQU VOUT=(C.times.V.sub.BE +K.times.C.times..DELTA.V.sub.BE)/A * C
where K is capacitive ratio of capacitors C1 and C2; A is the capacitive ratio of C3 and C2, and; C is the capacitive value of C2.
Although the bandgap reference circuit of FIG. 2 eliminates the transistor mismatch problem of the Brokaw reference circuit, its suffers from its own inaccuracies due to the switch impedances as well as the variations in the capacitors. In addition, the bandgap reference circuit of FIG. 2 requires a three-phase clock which adds complexity to the circuit. Accordingly, what is desired is a simplified switch capacitor bandgap reference circuit.